Processes for the preparation of stable aqueous polyurethane polyurea dispersions are already known and described, for example, in German Pat. Nos. 1,184,946 and 1,178,586; German Auslegeschrift No. 1,237,306, German Offenlegungsschriften No. 1,495,745; 1,595,602; 1,770,068 and 2,019,324; D. Dieterich et al, Angew. Chem. 82, 53 (1970) add U.S. Pat. Nos. 3,686,108; 3,756,992. The dispersions which have been described are based on the principle of incorporating hydrophilic centers in a macromolecular chain of a polyurethane-polyurea molecule. These hydrophilic centers or so-called internal emulsifiers in the known dispersions are ionic groups or ether functions. The ionic groups are either incorporated in the prepolymer in the form of special diols or used as modified amines for chain lengthening the prepolymers which each have at least two isocyanate end functions.
High quality polyurethane films suitable, for example, for textile coating have hitherto been obtained from dispersions which are prepared with the use of organic solvents in the polyaddition reactiton.
By carrying out the polyaddition in solution, it is possible to build up a high quality polyurethane in a homogeneous phase before it is dispersed in water. The solid constituent of the dispersed particles is, therefore, also highly homogeneous.
This highly developed process, which includes anionic and cationic and non-ionic emulsifier segments, has the disadvantage that the organic solvent must be distilled off and rectified by a complicated and expensive process. This results in a poor volume/time yield in the manufacturing process. The organic solvent used as reaction medium increases the risk of explosion and fire during the manufacturing process. When attempts are made to produce such products without the aid of organic solvents, one obtains at best relatively coarse dispersions whose film forming ability and mechanical properties are insufficient for many purposes.
Processes for the solvent-free preparation of polyurethane dispersions have also become known, e.g. the so called melt dispersion process as described in German Offenlegungsschrift No. 1,770,068, U.S. Pat. No. 3,756,992 and D. Dieterich and H. Reiff, Angew, makromol. Chem. 76, 85 (1972). In this process, an oligourethane which has been modified with ionic groups and contains acylated amino end groups is converted into the corresponding oligourethane containing methylol end groups attached to acylated amino groups. This oligourethane is then chain-lengthened by a heat treatment which brings about a condensation reaction of the reactive methylol end groups. This chain lengthening may be carried out in the presence of water so that an aqueous dispersion of a polyurethane is directly obtained. This process is particularly suitable for the preparation of cationically modified polyurethanes or of polyurethanes containing anionic carboxylate groups. The required combination of the isocyanate polyaddition reaction with the aforesaid chain lengthening reaction via methylol groups which are attached to acylamino end groups and capable of undergoing polycondensation reactions makes this process more difficult and complicated than the conventional isocyanate polyaddition by the prepolymer process in which isocyanate prepolymers are reacted with conventional chain lengthening agents such as water or diamines. This additional expenditure could be justified in the process according to German Offenlegungsschrift No. 1,770,068 and U.S. Pat. No. 3,756,992 on the grounds that it was thereby possible, for the first time, to produce polyurethane dispersions without the aid of special stirrers and without the aid of either emulsifiers or solvents.
Another process for the solvent-free preparation of polyurethanes in aqueous dispersions has been described in German Offenlegungsschrift No. 2,446,440 and allowed U.S. Patent Application Ser. No. 777,206 filed Mar. 14, 1977, now U.S. Pat. No. 4,108,814. In this process, a prepolymer containing sulphonate groups and isocyanate groups is reacted without the aid of solvents with a compound which contains isocyanatereactive hydrogen atoms and is at least difunctional in the isocyanate polyaddition reaction.
In this process, the reaction of the isocyanate prepolymer with the isocyanate-reactive compound occurs during the dispersion process, i.e. after the chain lengthening agent has been dissolved in the water of dispersion or is added after the prepolymer has been dispersed in water. The said process makes it possible for high quality polyurethanes containg sulphonate groups to be obtained in the form of aqueous dispersions. It has, however, the disadvantage that it is restricted to polyurethanes which contain sulphonate groups and that only water or water-soluble polyamines can be used as chain lengthening agents.
Another characteristic feature of this process is that chain lengthening, i.e. the building up to a high molecular weight polyurethane urea, takes place in a heterogeneous phase. The chain lengthening agent e.g. diamine in water, is in the liquid phase and it is only after a certain length of time that it reaches the center of the dispersed particles. It is clear that the chain lengthening agent, i.e. the diamine dissolved in water, will first undergo reaction in the ouer regions of the dispersed particle. The concentration of chain lengthening agentn decreases progressively towards the center of the particles. This process therefore gives rise to dispersions in which the solid particles are not homogeneous. The polyurethane ureas obtained, therefore, have less mechanical strength and in some cases, less resistance to hydrolysis than products which have been prepared in homogeneous solution.
In addition, it is almost impossible to prepare highly concentrated dispersions with a solids content of more than about 40% by the above mentioned process because, at higher solids concentrations, the individual dispersed particles are linked together into larger lumps by the chain lengthening agent which is present mainly between the particles. This results in very high viscosities, which render the products unsuitable for further processing, and in some cases even convert the aqueous dispersion into a paste. Another disadvantage is that polyamines which are insoluble or only sparingly soluble in water cannot be used as chain lengthening agents in the described process, but it is precisely with these polyamines, e.g. diaminocyclohexylmethane, that polyurethane ureas with excellentn mechanical properties can be obtained.
It was, therefore, an object of the present invention to provide a solvent-free or low solvent process by which polyurethanes in the aqueous phase could be obtained with improved properties. To achieve this improvement, the high molecular weight solid lwhich is distributed in the liquid aqueous medium, was to be obtained in as homogeneous a phase as possible.
It has now surprisingly been found that exceptionally high quality polyurethanes in aqueous dispersion or solution could be obtained when prepolymers which, in addition to isocyanate end groups, contain a group which renders them hydrophilc and/or external emulsifier, are mixed with an at least partly blocked polyamine chain lengthening agent before they are dispersed in water.